Digestive System: Gastrointestinal Tract
Time period:
days 21–50
The gut tube
forms when the yolk sac is pulled into the embryo and pinched off (see Figure
20.2) as the flat germ layers of the early embryo fold laterally and
cephalocaudally (head to tail). Consequently, it has an endodermal lining
throughout with a minor exception towards the caudal end. Epithelium forms from
the endoderm layer and other structures are derived from the mesoderm.
Initially,
the tube is closed at both ends, although the middle remains in contact with
the yolk sac through the vitelline duct (or stalk) even as the yolk sac
shrinks (Figure 33.1).
The cranial
end will become the mouth and is sealed by the buccopharyngeal membrane,
which will break in the fourth week, opening the gut tube to the amniotic
cavity. The caudal end will become the anus and is sealed by the cloacal
membrane, which will break during the seventh week.
Buds develop
along the length of the tube that will form a variety of gastrointestinal and
respiratory structures (see Chapter 34).
The gut is
divided into foregut, midgut and hindgut sections by the region of the gut tube
that remains linked to the yolk sac and by the anterior branches from the aorta
that supply blood to each part (Figure 33.2).
The foregut
will develop into the pharynx, oesophagus, stomach and the first two parts of
the duodenum to the major duodenal papilla, at which the common bile duct and
pancreatic duct enter. The midgut includes the remainder of the duodenum and
the small and large intestine through to the proximal two‐thirds of the
transverse colon. The hindgut includes the distal third of the transverse colon
and the large intestine through to the upper part of the anal canal.
Each
division of the gut is supplied by a different artery. The foregut is supplied
by branches from the coeliac artery directly from the descending aorta.
The midgut receives blood from the superior mesenteric artery and the
hindgut from the inferior mesenteric artery (Figure 33.2).
The foregut
grows in length with the embryo, and epithelial cells proliferate to fill the
lumen. The tube is later recanalised and only becomes a squamous epithelium
during the foetal period. Failure of this normal process causes problems of
stenosis (narrowing) or atresia (blocked) in the oesophagus or duodenum.
Part of the
foregut tube begins to dilate in week 4, the dorsal side growing faster than
the ventral side until week 6. This will become the stomach, and the dorsal
side becomes the greater cur- vature. The dorsal mesentery (dorsal
mesogastrium) will expand significantly to form the greater omentum.
The stomach
rotates to bring the left side around to become the ventral surface, explaining
why the left vagus nerve innervates the anterior of the stomach (Figure 33.3).
This rotation also moves the duodenum into the adult C‐shaped position.
The midgut
also lengthens considerably, looping and twisting as it does so, filling the
abdominal cavity. At approximately 6 weeks the midgut grows so quickly there is
not enough room in the abdomen to contain it, and it herniates into the
umbilical cord (Figure 33.4). The midgut also rotates through 270° counterclockwise
(if you were to be looking at the abdomen), bringing the developing cae-
cum from the inferior abdomen up the left of the developing small intestine to
the top of the abdomen, and around to descend to its adult location in the
lower right quadrant. The axis of this rotation is the superior mesenteric
artery and the rotation is of particular significance when considering the
layout of the small and large intestines and accessory organs in adult anatomy.
The midgut
re‐enters the abdomen in week 10, and it is thought that growth of the abdomen
together with regression of the mesonephric kidney and a reduced rate of liver
growth are important factors in this occurring normally.
The last
part of the gut tube, the hindgut, ends initially in a simple cavity called the
cloaca. The cloaca is also continuous with the allantois, a remnant of the yolk
sac that largely regresses but contributes to the superior parts of the bladder
in the human embryo.
A wedge of
mesoderm, the urorectal septum, moves caudally towards the cloacal
membrane as the embryo grows and folds during weeks 4–7 (Figure 33.5). The
urorectal septum divides the cloaca into a primitive urogenital sinus anteriorly
and an anorectal canal posteriorly. The urogenital sinus will form parts
of the bladder and the urogenital tract.
The cloacal
membrane ruptures in the seventh week, opening the gut tube to the amniotic
cavity. The caudal part of the lining of the anal canal is thus derived from
ectoderm and the cephalic part from endoderm. Subsequently, the caudal part of
the anal canal receives blood from branches of the internal iliac arteries and
the cephalic part receives blood from the artery of the hindgut, the inferior
mesenteric artery. Similarly, portosystemic anastomoses also occur here.
Mesenteries
of the gut form as a covering of mesenchyme passing over the gut tube from the
posterior body wall of the embryo when the tube is in close contact with it.
With growth the gut tube moves further into the abdominal cavity and away from
the posterior wall. A bridging connective tissue forms suspending the gut and
its asso- ciated organs within the abdomen in a dorsal mesentery for most of
its length and a ventral mesentery around the lower foregut region. The ventral
mesentery is derived from the septum transversum.
The dorsal
mesentery will form the mesenteries of the small and large intestines of the
adult gastrointestinal tract, and also forms the greater omentum (Figure
33.6). The ventral mesentery will form the lesser omentum between the
stomach and the liver, and the falciform ligament between the liver and the
anterior abdominal wall.
The
extensive lengthening and rotation of the midgut causes the dorsal mesentery to
become considerably larger and more convoluted, and its initial simplicity explains
the short diagonal attachment of the mesentery of the small intestine to the
posterior abdominal wall in the adult. When the hindgut finds its final
position in the foetus the mesenteries of the ascending and de colon fuse with
the peritoneum of the posterior body wall.
